Grids and Grid Files - Basic Concepts

A grid is a lattice of regularly spaced data points superimposed on a projection
of the earth. Grids can store numerical model data and serve as an interface
between observational data stored in MD files and analyzed data displayed as
contours on a frame. To contour observational data, compute derived parameters,
and display manually digitized radar (MDR) data, etc., the data must first be
interpolated from the observing locations to a uniform latitude, longitude lattice.
McIDAS stores this geographic lattice as a grid.

Grids are stored in grid files, GRIB files or netCDF files. This manual will focus mainly on grid files because it is a format designed specifically for McIDAS and it is the format used when copying or modifying grids. Grid files use the naming convention GRnnnnnn where nnnnnn is
the six-digit file number. If the file number is less than six digits, I and
D replace the first two digits and zeros precede the number. For example, GR111111
is the name of the file for grid 111111, GRI90112 is the name for grid file 90112,
and GRID0013 is the name for grid file 13. In ADDE, sequences of grid files are
grouped together in a dataset.

Each grid file contains a grid file directory and up to 10,000 grids. Each grid
contains a grid header and grid data, as shown below. A sample grid dataset directory
and a listing of grid header information are also shown below.

The GRDLIST and GRDCOPY commands
manipulate individual grids or groups of grids. GRDLIST can
list grids, while GRDCOPY can
copy, move, and perform mathematical operations on grids. These operations
include adding or subtracting two grids, filling a grid with a constant value,
or redefining all values within a grid that are less than a specified number
to be a new number (e.g., all values less than zero are set to zero). If you
have u and v component grids, you can create a vorticity or divergence grid,
or use the grid components to advect a parameter in another grid.

The GRDINFO command provides
information about grids. You can use it to list information about the grid
header and statistics on the grid data, or list data points.

The GRDLIST and GRDCOPY commands
can also manipulate grid files. With these commands you can create, list, or
copy a grid file.

SSEC receives real-time grid data for many numerical models via the NOAAPORT data stream. The models and datasets change frequently so the following list is just an example of grid dataset names and models that are available.